[test] Add test for behavior when picker variant changes

[eps1lon]

Helps with #26123

Right now I'm probably moving forward with #26123 even though it does regress in certain areas. Since the behavior is already fairly inconsistent (notice how the displayed view changes when switching to mobile i.e. it goes from year to calendar view) I feel safer just resetting entirely instead of only persisting some areas (e.g. open is persisted but not view).

Long term we can probably bring this back by using controlled/uncontrolled patterns. Short term people can restore the previous behavior by controlling open:

function ResponsiveDateTimePicker() {
  const [open, setOpen] = React.useState(false);

	return <DateTimePicker open={open} onClose={() => setOpen(false)} onOpen={() => setOpen(true)} />
}

[mui-pr-bot]

No bundle size changes

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